Thermal switch device



June 23, 1964 s. P. MITOFF ETAL 3,138,686

THERMAL SWITCH DEVICE Filed Feb. 1, 1961 lll /2 In venz'ror's: Stephan 7? Mitoc,

Ro er' H. pr' by @WQ ,11: The/r A fito r'rz ey.

United States Patent O d3,13%,686 n n THERMAL SWITCH DEVICE Stephan P. Mitofr, Elnora, and Robert H. Pry, Schenectimely, N.Y., assignors to General Electric Company, a

corporation ofNewrYorlg y Filed Feb. 1, 1961, Ser. No. 86,546 Claims. (Cl. 20G- 142) This invention relates to thermal switch devices and more particularly to thermal switch devices with an abrupt, significant increase in resistance at a specific temperature.

A heating element is described in U.S. LettersV Patent 2,861,163 issued November 18, 1958, which includes a core of partially conductive material and a covering of 'a dielectric material. The core consists of 40 to 75 volume percent of conductive particles which may be of carbon including carbon black, copper, aluminum, silicon, silicon carbide, lead sulfide, iron sulfide, or molybdenum sulfide, and of a thermal cxpansible, non-conductive material which may be of Wax, parain, polyethylene, or polysiloxane. As the temperature of the core is increased, the non-conductive material undergoes a substantial increase in its volumetric displacement to spread the conductive particles apart to increase resistance to current flow and regulate the current and wattage through the element.

It would be 'desirable to have a thermal switch device which would operate to provide thermal and over-tem'- peiature detection and control, and current limitation control. For example, there has been a long-standing need by electric motor manufacturers for a dependable over-temperature thermal switch device which could be inserted in motor windings or used in series with Sa relay coil Vto prevent and control failure due to insulation breakdown. Such a device must switch abruptly below the temperature of decomposition or combustion of the insulation. Furthermore, a thermal switch device operable at a Vtemperature of 119 C. is of widespread interest for applications involving the lprotection of Class A insulation. Thus, it would be desirable to provide a thermal switch device with an abrupt, significant increase in resistance at a specific temperature for various applications.

It is an object of our invention to provide a thermal switch device With components in the solid state below the switching temperature which device changes its electrical resistance abruptly and significantly at a specific temperature,- the melting point of the insulating material of the devices resistor material, without apsignicant increase in volumetric displacement of its resistor material.

It is another object of our invention to provide a thermal switch device in which the temperature at which the abrupt and significant electrical resistance change occurs is established by the incorporation of a particular solid, crystalline insulating material in the resistor material of the device.

In carrying out our invention in one form, a thermal switch device comprises a pair of electrodes having leads attached thereto, yresistor material positioned between the electrodes, the resistor material comprising a solid, crystalline insulating material with a 'specific Vmelting point, and iinely divided, conductive-material concentrated at the grain boundaries of the "crystals of the insulating material, the crystals of the solid crystalline insulating material excluding 'the finely divided, conductive material upon solidification, and concentrating the conductive material at its grain boundaries, and an insulating member encapsulating at least the edges of the device.

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These and various other objects, features, and advantages of the invention will be better yunderstood from the following description taken in connection with the accompanyi-ng drawing in which:

FIGURE l is a partial sectional view of a thermal switch device embodying our invention;

FIGURE 2 is a partial sectional View of a modified thermal switch device; and

FIGURE 3 is a partialsectional view of the device of FIGURE l in which insulating member encapsulates the edges of the device. y

In FIGURE l of the drawing, a thermal switch `device is shown generally at 10 which comprises a pair of electrodes 11 with leads l2 spot-welded thereto andY resistor material 13 positioned between and in contact with electrodes 151. yResistor material 13 and its associated electrodes 11 are encapsulated in an insulating Amember 14 which covers also aportion of leads 512. Material 13 comprises a solid mixture of finely divided, conductive material, such as carbon black and 'a solid, `crystalline insulating material, such as sulphur. Electrodes 11 are preferably made of a material, such as nickel, tungsten, or platinum which have a high conductive interface with the resistor material. `Encapsulating `member 14 is a semirigid, insulating, and non-reactive material, such as an epoxy resin having good physical strength in a thin cross-section or includes at least an insulating and nonreacting coating.

In FIGURE 2 ofthe drawing, there is shown a modified M*thermal switch device which includes a pair of electrodes 11 having leads y12 welded thereto and :resistor material 15 positioned therebetween. Material 15 comprises a porous spacer of asbestos cloth impregnated with carbon black and sulphur. Electrodes 11 and spacer 15 are encapsulated in an insulating member 14 which 'covers also a portion of `leads 12.

In FIGURE 3 of the drawing, 'there is yshown the device of FIGURE l in which a pair of electrodes 11 having leads 12 welded thereto have resistor material 1'3 positioned therebetween. Insulating member 14 encapsulates the edges of the device.

We found unexpectedly that a thermal switch device could be constructed which included a pair of electrodes having leads attached thereto, resistor material therebetween, and an insulating member encapsulating at least the edges of the device. We found further that the resistor material required a mixture of lfinely divided, conductive material and a material which was solid, crystalline and insulating below vthe switching point to provide an abrupt, significant change in electrical resistance at a predetermined specilic temperature without -a significant increase -involu'metric displacement. lIt is necessary that vthe insulating material of the resistor 'material have a definite melting point at a specific temperature which is the desired switching temperature. It is also Vnecessary that the crystals of the solid, crystalline Iinsulating material exclude the resistors conductive material upon solidification and concentrate the conductive particles at its grain boundaries. The insulating lmaterial must be non-reactive with and a non-'solvent for the conductive material of the resistor material. We `found thatvsuitable insulating materials included sulphur, ice, succinic .anhydride, tribromobenzene, and 'benzoin Of these materials, sulphur in reason-ably `pure `form appeared to be the most desirable material for use in va thermal switch device involving the protection of Class A insulation since sulphur has a definite melting point of '119 C.

The conductive material of the resistor material must be finely divided, non-reactive with the insulating material, conductive, and have a density close to the insulating material or be in colloidal particle size. We found that the most desirable material was carbon black although acetylene black, lamp black, and a number of inely divided graphites were suitable. Additionally, colloidal particles of tungsten, cobalt, and nickel can be used as a conducting material. However, we discovered that the use of such metallic powders with particles larger than colloidal size provided a density mismatch whereby the particles separated out of the molten sulphur at the switching temperature rendering their incorporation valueless. We found also that a small weight percent range, such as 3 to 4 percent of conductive material was the optimum concentration in the resistor material considering both a low resistance at low temperature and a large resistance differential between low and high temperatures.

The electrodes and lead material must be chemically inert with the insulating material of the resistor material. Otherwise, a reactive product will form over a period of time which consumes and destroys the electrodes and leads. We have found that nickel, tungsten, and platinum are compatible with the above insulating materials. However, copper electrodes react with these insulating materials and are destroyed eventually.

The encapsulating member should be semi-rigid, strong in a cross-section, a good electrical insulator, a good thermal conductor, inert chemically to the insulating material of the resistor material and thermally stable under operating temperature or include at least a coating with these properties. Since some of these desirable properties are mutually incompatible, an encapsulating member containing the most desirable features should be employed. Of the materials tested, we found that a number of epoxy resins were desirable. For example, a preferred encapsulating member is Hysol Epoxi-Patch which is a smooth, non-flow epoxy resin paste manufactured by Hysol Corporation, Olean, New York. Additionally, low melting point glasses can be used for an encapsulating member. The edges of the thermal switch device or the entire device, including a portion of the leads can be encapsulated.

As shown in FIGURE 1 of the drawing, a pair of nickel electrodes 11 have nickel leads 12 welded thereto. Resistor material 13 is formed from a melt of sulphur to which is added 3 to 4 weight percent of inely divided carbon black. The resistor material is placed on one of the electrodes and the other electrode is positioned thereon. Upon cooling, the sulphur crystals exclude the carbon particles which concentrate at the grain boundaries. Through compression and orientation of the carbon at the grain boundaries, resistor material 13 becomes a good conductor. After the resistor material has solidified, a member 14 consisting of a thin layer of epoxy resin encapsulates electrodes 11, resistor material 13 and a portion of leads 12 to form a complete thermal switch device. Member 14 positions lirmly electrodes 11 which is important when the sulphur is made molten subsequently. The leads of the thermal switch device are connected, for example, to a battery (not shown) to provide a current through device 10. The device will exhibit a low resistance until the temperature therein reaches 119 C., the melting point of sulphur, when there will be an abrupt, signiiicant increase in resistance without a significant increase in volumetric displacement of the insulating material of the resistor material. This change occurs upon the melting of the sulphur within device 10 whereupon the carbon particles are dispersed into the sulphur resulting in a poor conductor. Upon cooling, the sulphur crystals exclude the carbon particles which concentrate at the grain boundaries to provide a good conductor. An abrupt resistance decrease does not occur until the ternperature is lowered from about 10 C. to 30 C. below the sulphur melting point. This appears to bedue to supercooling of the sulphur before solidilication takes place.

As shown in FIGURE 2 of the drawing, a pair of nickel electrodes 11 have nickel leads 12 welded thereto. Resistor material 15 is formed of a porous spacer, such as a piece of asbestos cloth, which is impregnated with molten sulphur and 3 to 4 weight percent of carbon black. After the mixture solidfies on the cloth which has a degree of compression, the spacer material is inserted between electrodes 11. Asbestos paper, cotton gauze, and glass cloth are also suitable spacers for impregnation. The spacer must be porous, an electric insulator, chemically inactive with sulphur, and thermally stable at operating temperatures. Member 14 consisting of an epoxy resin encapsulates electrodes 11, resistor material 15, and part of leads 12. A battery (not shown) is connected to leads 12 to produce a current through the thermal switch device. A low resistancev is exhibited by the device until the temperature therein reaches 119 C. at which time there is an abrupt, signilicant increase in resistance without a significant increase in volumetric displacement of the insulating material of the resistor material. This change is caused by the melting of the sulphur and the dispersion of the carbon particles therein providing poor conduction in the resistor material. Upon cooling, the sulphur crsytals exclude the carbon particles which concentrate at the grain boundaries to provide a good conductor. The abrupt resistance decrease does not occur until the temperature is lowered about 10 C. below the sulphur melting point. This supercooling elect can be very advantageous since the equipment in which the thermal switch device is used will not turn on until the temperature has dropped substantially below the turnolf temperature.

As shown in FIGURE 3, the device of FIGURE 1 has its edges encapsulated by insulating member 14. If desired, the device of FIGURE 2 can also have only its edges encapsulated in a similar manner. Otherwise, the operation of such thermal switch device in FIGURE 3 is identical with the operation of the device shown in FIG- URE 1.

Table I discloses resistor materials of sulphur and carbon black in which the weight percentage of carbon black is varied. Table II discloses the average specific resistance in ohm centimeters of the resistor materials in Table I at both an average low temperature of approximately 70 C. and at a temperature of 119 C., the melting point of sulphur. It will appreciated that an increase in electrode area and a decrease in electrode spacing will increase the current carrying capacity of the resistor material.

TABLE I Resistor Material (l) Sulphur-1% carbon black (2) Sulphur-2% carbon black (3) Sulphur-3% carbon black (4) Sulphur-4% carbon black (5) Sulphur-5% carbon black TABLE II Average Specific Resistance (Ohm Centirneters) Average Low High Tern- Temperature perature ot of Approxiof 120 C mately 70 C.

Several examples of forming thermal switch devices in accordance with the present invention are as follows:

EXAMPLE I A thermal switch device is constructed of a pair of platinum electrodes having platinum leads attached thereto. Each electrode has dimensions of five centimeters by one centimeter. The electrodes are spaced apart one centimeter between which is positioned resistor material comprising a mixture 'of three weight percent carbon and sulphur. The `device lis surrounded by a glass envelope. At a temperaturebelo'w 119 C., a typical resistance is tw`o ohms. lAbove 119 C., the resistance changes abrupt- 1y and significantly to 400 ohms.

EXAMPLE 1I yA thermal switch device is constructed of a pair of platinum electrodes having platinum leads attached thereto. Each electrode ,has dimensions of yfive centimeters by one centimeter. The electrodes are spaced apart one centimeter between which is positioned resistor material comprising a mixture of three weightl percent carbon and benzoin. The device is surrounded by a glass envelope. At a temperature below 137 C., a typical reist'an'ce is seven ohms. Above 137 C., the resistance changes abruptly and significantly to 200 ohms.

EXAMPLE III A thermal switch device is constructed of a pair of platinum electrodes having platinum leads attached thereto. Each electrode has dimensions of five centimeters by one centimeter. The electrodes are spaced apart one centimeter between which is positioned resistor material comprising a mixture of two weight percent carbon and succinic anhydride. The device is surrounded by a glass envelope. At a temperature below 185 C., a typical resistance is ten ohms. Above 185 C., the resistance changes abruptly and significantly to 200 ohms.

EXAMPLE IV A thermal switch device is constructed of a pair of nickel electrodes having nickel leads attached thereto. Each electrode is a circular disk having a diameter of 1A inch. The electrodes are spaced 0.050 inch apart between which is positioned resistor material comprising a porous, chemically inactive, insulating spacer of asbestos cloth impregnated with a mixture of three weight percent carbon and sulphur. The device is encapsulated with an epoxy resin. At a temperature below 119 C., a typical resistance is ohms. Above 119 C., the resistance changes abruptly and significantly to 400 ohms.

EXAHWIP'LE V A thermal switch device is constructed of a pair of platinum electrodes having platinum leads attached thereto. Each electrode has dimensions of live centimeters by one centimeter. The electrodes are spaced apart one centimeter between which is positioned resistor material comprising a mixture of four weight percent carbon and ice. The device is surrounded by a glass envelope. At a temperature below 0 C., a typical resistance is 100 ohms. Above 0 C., resistance changes abruptly and significantly to 16,000 ohms.

EXAMPLE VI A thermal switch device is constructed of a pair of platinum electrodes having platinum leads attached thereto. Each electrode has dimensions of five centimeters by one centimeter. The electrodes are spaced apart one centimeter between which is positioned resistor material comprising a mixture of one weight percent carbon and tribromobenzene. The device is surrounded by a glass envelope. At a temperature below 120 C., a typical resistance is four ohms. Above 120 C., the resistance changes abruptly and significantly to 200 ohms.

While other modifications of this invention and variations of structure which may be employed within the scope of the invention have not been described, the invention is intended to include such that may be embraced within the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A thermal switch device comprising a pair of electrodes la lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline. insulating material with a specific melting point, and finely divided, conductive material concentrated at the grain boundaries of the crystals of said insulating material, the crystals of said Vsolid,A crystalline insulating lmaterial excluding said finely divided, conductive material upon solidification and concentrating sa-id conductive material at its grain boundaries, and Yan Yinsulating member encapsulating at least the edgesA of said device. c l

2. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline insulating material with a specific melting point, and finely divided, conductive carbonaceous material concentrated at `the grain boundaries of the crystals ofrsaid insulating material, the crystals of said solid, crystalline insulating material excluding said finely divided, conductive material upon solidifieation and concentrating `said conductive material at its grain boundaries, and an linsulating member encapsulating at least the edges of said device. v

3. A thermal switch device comprising a pair of electrodes, la lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline insulating material of sulphur and finely divided, conductive carbonaceous material concentrated at the grain boundaries of the crystals of said sulphur, the crystals of said sulphur excluding said finely divided, conductive material upon solidication, and concentrating said conductive material at its grain boundaries, and an insulating member encapsulating at least the edges of said device.

4. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline insulating material of benzoin and finely divided, conductive carbonaceous material concentrated at the grain boundaries of the crystals of said benzoin, the crystals of said benzoin excluding said finely divided, conductive material upon solidiication, and concentrating said conductive material at its grain boundaries, and an insulating member encapsulating at least the edges of said device.

5. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline insulating material of succinic acid and finely divided, conductive carbonaceous material concentrated at the grain boundaries of the crystals of said succinic acid, the crystals of said succinic acid excluding said finely divided, conductive material upon solidification, and concentrating said conductive material at its grain boundaries, and an insulating member encapsulating at least the edges of said device.

6. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline insulating material of tribromobenzene and finely divided, conductive carbonaceous material concentrated at the grain boundaries of the crystals of said tribromobenzene, the crystals of said tribromobenzene excluding said finely divided, conductive material upon solidification, and concentrating said conductive material at its grain boundaries, and an insulating member encapsulating at least the edges of said device.

7. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline insulating material with a specific melting point, and finely divided, conductive material concentrated at the grain boundaries of the crystals of said insulating material, the crystals of said solid, crystalline insulating material excluding said nely divided, conductive material upon solidication and concentrating said conductive material at its grain boundaries, and an insulating member encapsulating said device.

8. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a porous, chemically inactive, insulating spacer a solid, crystalline insulating material with a specific melting point, and nely divided, conductive material concentrated at the grain boundaries of the crystals of said insulating material impregnated therein, the crystals of said solid, crystalline insulating material excluding said finely divided, conductive material upon solidication and concentrating said conductive material at its grain boundaries, and an insulating material encapsulating said device.

9. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a solid, crystalline insulating material with a specific melting point, and nely divided, conductive material concentrated at the grain boundaries of the crystals of said insulating material, the crystals of saidv solid, crystalline insulating material excluding said nely divided, conductive material upon solidification and concentrating said conductive material at its grain boundaries, and an insulating member encapsulating the edges of said device. i

10. A thermal switch device comprising a pair of electrodes, a lead attached to each of said electrodes, resistor material positioned between said electrodes, said resistor material comprising a porous, chemically inactive, insulating spacer, a solid, crystalline insulating material of sulphur, and iinely divided, conductive carbonaceous material concentrated at the grain boundaries of the crystals of said insulating material impregnated therein, the crystals of said sulphur excluding said iinelydivided, conductive material upon solidication, and concentrating said conductive material at its grain boundaries, and an insulating member encapsulating at least the edges of said device.

References Cited in the tile of this patent UNITED STATES PATENTS 

1. A THERMAL SWITCH DEVICE COMPRISING A PAIR OF ELECTRODES, A LEAD ATTACHED TO EACH OF SAID ELECTRODES, RESISTOR MATERIAL POSITIONED BETWEEN SAID ELECTRODES, SAID RESISTOR MATERIAL COMPRISING A SOLID, CRYSTALLINE INSULATING MATERIAL WITH A SPECIFIC MELTING POINT, AND FINELY DIVIDED, CONDUCTIVE MATERIAL CONCENTRATED AT THE GRAIN BOUNDARIES OF THE CRYSTALS OF SAID INSULATING MATERIAL, THE CRYSTALS OF SAID SOLID, CRYSTALLINE INSULATING MATERIAL EXCLUDING SAID FINELY DIVIDED, CONDUCTIVE MATERIAL UPON SOLIDIFICATION AND CONCENTRATING SAID CONDUCTIVE MATERIAL AT ITS GRAIN BOUNDARIES, AND AN INSULATING MEMBER ENCAPSULATING AT LEAST THE EDGES OF SAID DEVICE. 